Always call `find_package(MPI)`.

Always call `find_package(MPI)`.

It is getting increasingly hard to coordinate MPI requirements across
the toolkit that a GROMACS or its CMake infrastructure may be involved
in.

This change allows is to call `find_package(MPI)` exactly once in the
GROMACS CMake configuration and begins to normalize infrastructure on
the modern CMake FindMPI.cmake module, which has been substantially
updated since the last major revisions to gmxManageMPI.cmake.

Also, try to decouple and compartmentalize GMX_MPI, GMX_LIB_MPI, and
the presence of MPI in the build system.

Follow-up changes can continue to refine the dependent use cases and
allow more coordination of the different MPI requirements in different
build configurations.

Refs #3672
Also refs #3776